1. Field of the Invention
The disclosures herein relate generally to personal computer systems and more particularly to personal computer systems employing DC to DC power conversion.
2. Description of the Related Art
Personal computer systems in general and Intel/Microsoft compatible personal computer systems in particular have attained widespread acceptance. The term compatible is used to denote those computer systems employing microprocessor and chip set hardware supplied by Intel Corporation and operating system software supplied by Microsoft Corporation. These personal computer systems now provide computing power to many segments of today's modem society. A personal computer system can usually be defined as a desktop, floor-standing, or portable microcomputer that includes a system unit having a system processor with associated volatile and non-volatile memory, a display monitor, a keyboard, one or more floppy diskette drives, a mass storage device, an optional CD-ROM or DVD drive and an optional printer. One of the distinguishing characteristics of these systems is the use of a system board or motherboard to electrically connect these components together. These personal computer systems are information handling systems which are designed primarily to provide independent computing power to a single user, (or a relatively small group of users in the case of personal computers which serve as server systems.)
Personal computer systems typically include a power supply which converts AC mains power (120 volts in the United States, and 220 volts in many other countries) down to a smaller DC voltage useful for supplying the various components of the computer system. Different components of the computer system often have different DC voltage requirements. For example, the power rail which supplies an appropriate DC voltage to the microprocessor of the computer system may have one voltage requirement. The L2 cache associated with the microprocessor may have another voltage requirement while the system bus and peripherals may have still other voltage requirements. Personal computer systems typically include several DC to DC converters to down convert one DC voltage from the power supply to respective voltages used by the particular components of the computer system.
A conventional switched mode DC to DC buck converter useful for this purpose is depicted in FIG. 1 as converter 10. Converter 10 includes a switching transistor Q1 to which an input DC voltage V.sub.IN is provided. A free-wheeling diode D1 is coupled between the emitter of switching transistor Q1 and ground. A large inductor L, with a inductance of 5 to 10 .mu.H for example, is coupled between load 15 and the juncture of the transistor Q1 emitter and diode. The output rail of the converter is designated as output rail 20 and the output voltage V.sub.OUT is generated at this rail. A pulse width modulator (PWM) IC 25 is coupled between output rail 20 and the input or base of switching transistor Q1. PWM IC 25 senses the output voltage V.sub.OUT as the load 15 varies and appropriately adjusts the pulse width of the control signal applied to switching transistor Q1 to dynamically regulate the output voltage V.sub.OUT to the desired value. The operation of a buck converter similar to buck converter 10 is described in more detail in the publication "ARRL Handbook For Radio Amateurs", pages 6-27 through 6-28, published by the American Radio Relay League, the disclosure of which is incorporated herein by reference. The operation of another buck converter similar to buck converter 10 is described in "Switching Power Supply Design" by Pressman, published by McGraw Hill, pages 3-35, the disclosure of which is incorporated herein by reference. The switching frequency of switching transistor Q1 in conventional converter 10 is relatively low, for example 100 KHz in the present example.
During normal operation, the load presented to a DC to DC converter can vary from minor to major current fluctuations. For example, the DC to DC converter which supplies power to the microprocessor power rail will experience high frequency load fluctuations at the microprocessor switching rate which is in the MHz range. The average load current can be substantially constant with minor fluctuations or can change dramatically due to a change in microprocessor state condition such as transitioning from a sleep state to a fully active state (i.e. a major fluctuation).
Thus, the DC to DC converter in a computer system must be able to deal with both of the major and minor fluctuations in load current. The system bus and other buses of the computer system experience significant changes in operating current as well. Moreover, other I/O devices in the system, such as hard drives, floppy drives, CD ROMs, DVDs also present varying current requirements. It is important that a DC to DC converter be able to provide relatively constant DC output voltage as the load dynamically changes.
Conventional DC to DC buck converter 10 responds to load changes in the following manner. DC to DC buck converter 10 includes a plurality of transition control capacitors 30 which are collectively designated as C.sub.A. The C.sub.A capacitors are coupled between output rail 20 and ground as shown in FIG. 1. Typically, the C.sub.A capacitors are physically located adjacent load 15. The C.sub.A capacitors function as high frequency bypass capacitors which control the noise at rail 20 with respect to ground, such noise being due to the switching transitions of the microprocessor or other load 15 presented to the converter. The C.sub.A capacitors are typically low ESR (equivalent series resistance)/low ESL (equivalent series inductance) devices. For this reason, when charged, they are capable of maintaining the voltage at load 15 by becoming a source of current into load 15 when high frequency load transitions occur. This can occur because of the very low parasitic series resistive and inductive properties of this type of capacitor as compared to the C.sub.B capacitors discussed subsequently. The C.sub.A capacitors typically are relatively small capacitors such as 1 to 22 uF tantalum or ceramic capacitors.
To address major load fluctuations, such as when the load changes dramatically from 5A to 35A, converter 10 employs a plurality of capacitors 35, designated collectively as C.sub.B, which are coupled between output rail 20 and ground as shown. The C.sub.B capacitors typically are relatively large capacitors such as 820 uF to 3900 uF high performance aluminum electrolytic which often take the form of physically large silos. These C.sub.B capacitors function together as a bulk capacitor or output capacitor for the converter. When the C.sub.A transition control capacitors experience a transition at the microprocessor rate, they dump current into the load to control the transition. At the same time, the bulk capacitors C.sub.B start to replenish the energy of the C.sub.A transition control capacitors. This is one of the functions of the bulk capacitors C.sub.B. It is noted that for minor load variations as well, capacitor C.sub.B still replenishes capacitor C.sub.A except to a lesser magnitude according to the lesser needs of C.sub.A during a minor load fluctuation.
To summarize, the C.sub.A capacitors provide a substantial energy source during each microprocessor cycle (or other load cycle) independent of average microprocessor load. In between each microprocessor clock cycle there is energy replacement or replenishment from capacitors C.sub.B to capacitors C.sub.A at a somewhat lower transfer rate than the microprocessor consumption rate at microprocessor clock transitions. That replenishment rate is defined mostly by parasitic inductances within capacitors C.sub.B or physical implementation limitations such as printed circuit board (PCB) impedances or connector impedances between C.sub.B and C.sub.A. During a very large step load change the amount of energy replenishment from C.sub.B to C.sub.A will be proportional to the magnitude of the step load change.
FIG. 2A is a time line diagram of the load current, I.sub.LOAD, vs. time. It is observed that at transition 40 the load current rapidly increases as the microprocessor activities change. FIG. 2B is a time line diagram of the inductor current, I.sub.L, vs. time. The average rate of change for I.sub.L is determined by the PWM IC 25. The average rate of change of I.sub.L is considerably slower than the rate of change of I.sub.LOAD. FIG. 2C is a time line diagram of the current I.sub.CB in series with capacitor C.sub.B. It is noted that transition 45 is a slower transition than observed at transition 40 in FIG. 2B due to the parasitics observed and discussed earlier. During this discrepancy period, it is capacitor C.sub.A which is supplying energy to maintain the load current, I.sub.LOAD, thus maintaining the load voltage within a predetermined regulation.